1. Field of the Invention
The present invention relates to engine control and more particularly to a part of the air system control of a gasoline engine equipped with variable timing of VVT type (Variable Valve Timing) and with an external recirculation circuit for the low-pressure exhaust gas (EGR). Variable timing is a technology allowing several parameters to be varied in a combustion engine, notably timing, opening time and/or lift of the intake and exhaust valves.
2. Description of the Prior Art
In a gasoline engine, the amount of burnt gas present in the cylinders upon closure of the intake valve is obtained in two ways which are through internal burnt gas recirculation (IGR) via the intake and exhaust valve actuators, which are the variable timing means referred to as VVT actuators, and through external recirculation via the EGR circuit.
In fact, variable timing allows achieving internal recycling of the burnt gas by controlling opening and closing of the intake and exhaust valves. The exhaust gas flows back from the exhaust line to the intake line when the intake and exhaust valves are simultaneously open. The duration and the amplitude of the simultaneous valve opening is, in the case of internal gas recirculation, decisive for the amount of gas recycled. In this case, variable timing means make it possible for at least one intake valve to be controlled so as to be open before the piston top dead center in the cylinder, while at least one exhaust valve is controlled so as to be closed just before this piston top dead center. Both valves are then opened at the same time and the exhaust gas is recycled. The advantages of internal gas recycling as opposed to external recycling are the fast reaction of the system and good distribution of the recycled gas.
These two burnt gas sources have very distinct response times. Indeed, IGR is controlled through the phase shift of the valves, which is very fast. On the other hand, EGR control is very slow because the burnt gas flow time in the exhaust gas recirculation circuit is long, due to the length of this circuit. It seems pertinent to control the fast quantity (IGR via the VVT actuators) under transient conditions so as to make up for the EGR slowness (via the EGR valve). Indeed, for transient conditions, a fast response time is necessary.
A typical example of transient conditions is easing off the gas pedal under high load. At the start of the transient state, the EGR rate is high. In fact, the EGR rate is high under high load so as to push back the limits of engine knock. At the end of the transient state, under partial load, the rate is zero whereas the IGR rate increases. Indeed, the IGR rate is high under partial load due to the valve overlap with a view to decreasing engine pumping losses. However, under transient conditions, the amount of EGR entering the cylinders is not zero since burnt gas is present throughout the intake line. There is therefore a high risk of having too large an amount of burnt gas in the chamber (EGR plus IGR) and thus engine flameout. The goal, in this case of transient state, is then to limit as much as possible the amount of IGR until the intake line is emptied of the burnt gas it contains.
FIG. 3 shows the setpoint determination scheme for the air loop (10) of a gasoline engine with EGR and IGR according to the prior art. From an engine torque setpoint Tqsp and the engine speed measurement Ne, engine maps (MAP) allow defining the setpoints relative to the sucked air mass setpoint mairsp, the burnt gas fraction in the intake manifold Xintsp, positions of the intake and exhaust valve actuators VVTintsp and VVTexhsp for the steady state which are sought for the engine torque setpoint Tqsp. The air mass setpoint mairsp is then modified into a sucked mass setpoint maspsp using a value of the burnt gas fraction in the intake manifold Xint. This burnt gas fraction in the intake manifold can be obtained by any means, notably an estimation method or a measurement method. From the intake mass setpoint maspsp and from the measurement of the position of actuators (8 and 9) of the exhaust (13) and intake (12) valves, VVTexh and VVTint, a filling model (MR) allows giving a pressure setpoint in the intake manifold Pintsp.
Controllers (15) allow controlling the intake pressure, the EGR and variable timing means (8 and 9) to guarantee the pressure setpoint Pintsp, the burnt gas fraction setpoint in the intake manifold Xintsp and the positions of the intake and exhaust valve actuators VVTintsp and VVTexhsp of the steady state.
The method described in FIG. 3 thus does not allow controlling the burnt gas fraction in the cylinder under transient conditions.